Use only reflect.TypeOf to detect if argument is a string.

The wasString return is only needed in doPrint with the 'v' verb.
This type of string detection is handled correctly by reflect.TypeOf
which is used already in doPrint for identifying a string argument.

Remove now obsolete wasString computations and return values.

Change-Id: Iea2de7ac0f5c536a53eec63f7e679d628f5af8dc
Reviewed-on: https://go-review.googlesource.com/19976
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>